Flat panel array antenna technology may not be extensively used in the licensed commercial microwave point-to-point or point-to-multipoint market, where more stringent electromagnetic radiation envelope characteristics consistent with efficient spectrum management may be more common. Antenna solutions derived from traditional reflector antenna configurations, such as prime focus fed axi-symmetric geometries, can provide high levels of antenna directivity and gain at relatively low cost. However, the extensive structure of a reflector dish and associated feed may require enhanced support structure to withstand wind loads, which may increase overall costs. Further, the increased size of reflector antenna assemblies and the support structure required may be viewed as a visual blight.
Array antennas typically utilize printed circuit technology or waveguide technology. The components of the array that interface with free-space, known as the elements, typically utilize microstrip geometries, such as patches, dipoles, and/or slots, or waveguide components such as horns and/or slots. The various elements may be interconnected by a feed network, so that the resulting electromagnetic radiation characteristics of the antenna can conform to desired characteristics, such as the antenna beam pointing direction, directivity, and/or sidelobe distribution.
Flat panel arrays may be formed, for example, using waveguide or printed slot arrays in resonant or travelling wave configurations. Resonant configurations typically cannot achieve the desired electromagnetic characteristics over the bandwidths utilized in the terrestrial point-to-point market sector, while travelling wave arrays typically provide a mainbeam radiation pattern which moves in angular position with frequency. Because terrestrial point-to-point communications generally operate with go/return channels spaced over different parts of the frequency band being utilized, movement of the mainbeam with respect to frequency may prevent simultaneous efficient alignment of the link for both channels.
Corporate fed waveguide or slot elements may be used in the design of fixed beam antennas to provide desired characteristics. However, it may be necessary to select an element spacing which is generally less than one wavelength, in order to avoid the generation of secondary beams known as grating lobes, which may not meet regulatory requirements, and/or may detract from the antenna efficiency. This close element spacing may conflict with the feed network dimensions. For example, in order to accommodate impedance matching and/or phase equalization, a larger element spacing may be required to provide sufficient volume to accommodate not only the feed network, but also sufficient material for electrical and mechanical wall contact between adjacent transmission lines (thereby isolating adjacent lines and preventing un-wanted interline coupling/cross-talk).
The elements of antenna arrays may be characterized by the array dimensions, such as a N×M element array where N and M are integers. In a typical N×M corporate fed array, (N×M)−1 T-type power dividers may be employed, along with N×M feed bends and multiple N×M stepped transitions in order to provide acceptable VSWR performance. Feed network requirements may thus be a limiting factor in space efficient corporate fed flat panel antenna arrays.